Induction of defence response in peanut elicited by Bacillus sp. CHEP5: A biological strategy for control of smut disease caused by Thecaphora frezzii in the field.

Peanut is susceptible to many diseases; among them, peanut smut disease caused by Thecaphora frezzii is the most damaging, causing yield losses of 30%. Fungicide treatment is not effective to control this disease. In this scenario, biological control would be an alternative to diminish the disease. Systemic resistance induced by a biotic agent is known to be effective against a broad spectrum of pathogens. In this study we evaluated the effect of different inoculation strategies of Bacillus sp. CHEP5, a peanut native strain, on peanut smut incidence and severity in field experiments. Peroxidase activity and accumulation of phenolic compounds were measured as changes associated with induced defensive traits. After three consecutive field trials, we found that Bacillus sp. CHEP5 inoculation protects peanut from T. frezzii because incidence and severity were reduced in two field trials. Furthermore, bacterial inoculation in the furrow followed by foliar application around the date of peg development would be the best strategy to control the disease. In addition, a correlation was found between increase in plant phenolic content and decrease in smut disease parameters. Thereafter, we concluded that Bacillus sp. CHEP5 may reduce smut as a result of plant defence response induction.

Role of ethylene in effective establishment of the peanut-bradyrhizobia symbiotic interaction.

Ethylene has been implicated in nitrogen fixing symbioses in legumes, where rhizobial invasion occurs via infection threads (IT). In the symbiosis between peanut (Arachis hypogaea L.) and bradyrhizobia, the bacteria penetrate the root cortex intercellularly and IT are not formed. Little attention has been paid to the function of ethylene in the establishment of this symbiosis. The aim of this article is to evaluate whether ethylene plays a role in the development of this symbiotic interaction and the participation of Nod Factors (NF) in the regulation of ethylene signalling. Manipulation of ethylene in peanut was accomplished by application of 1-aminocyclopropane-1-carboxylic acid (ACC), which mimics applied ethylene, or AgNO3, which blocks ethylene responses. To elucidate the participation of NF in the regulation of ethylene signalling, we inoculated plants with a mutant isogenic rhizobial strain unable to produce NF and evaluated the effect of AgNO3 on gene expression of NF and ethylene responsive signalling pathways. Data revealed that ethylene perception is required for the formation of nitrogen-fixing nodules, while addition of ACC does not affect peanut symbiotic performance. This phenotypic evidence is in agreement with transcriptomic data from genes involved in symbiotic and ethylene signalling pathways. NF seem to modulate the expression of ethylene signalling genes. Unlike legumes infected through IT formation, ACC addition to peanut does not adversely affect nodulation, but ethylene perception is required for establishment of this symbiosis. Evidence for the contribution of NF to the modulation of ethylene-inducible defence gene expression is provided.

An oxidative burst and its attenuation by bacterial peroxidase activity is required for optimal establishment of the Arachis hypogaea-Bradyrhizobium sp. symbiosis.

AIMS: The main purpose of this study was to determine whether the Arachis hypogaea L. root oxidative burst, produced at early stages of its symbiotic interaction with Bradyrhizobium sp. SEMIA 6144, and the bacterial antioxidant system are required for the successful development of this interaction. METHODS AND RESULTS: Pharmacological approaches were used to reduce both plant oxidative burst and bacterial peroxidase enzyme activity. In plants whose H2 O2 levels were decreased, a low nodule number, a reduction in the proportion of red nodules (%) and an increase in the bacteroid density were found. The symbiotic phenotype of plants inoculated with a Bradyrhizobium sp. SEMIA 6144 culture showing decreased peroxidase activity was also affected, since the biomass production, nodule number and percentage of red nodules in these plants were lower than in plants inoculated with Bradyrhizobium sp. control cultures. CONCLUSIONS: We demonstrated for the first time that the oxidative burst triggered at the early events of the symbiotic interaction in peanut, is a prerequisite for the efficient development of root nodules, and that the antioxidant system of bradyrhizobial peanut symbionts, particularly the activity of peroxidases, is counteracting this oxidative burst for the successful establishment of the symbiosis. SIGNIFICANCE AND IMPACT OF THE STUDY: Our results provide new insights into the mechanisms involved in the development of the symbiotic interaction established in A. hypogaea L. a legume infected in an intercellular way.